An exhaust gas of automobiles which use gasoline for fuel contains hazardous components such as hydrocarbon (THC), carbon monoxide (CO), and nitrogen oxide (NOx). Therefore, it is necessary to purify each of the hazardous components in such a manner that the hydrocarbon (THC) is converted into water and carbon dioxide by oxidation; the carbon monoxide (CO) is converted into the carbon dioxide by oxidation; and the nitrogen oxide (NOx) is converted into nitrogen by reduction.
As a catalyst (hereinafter, referred to as an “exhaust gas purifying catalyst”) adapted to treat these exhaust gases, three way catalysts (TWC) capable of oxidizing and reducing CO, THC, and NOx have been used.
Three way catalysts are well known, in which a noble metal is supported on a refractory oxide porous body having a high-specific surface area, for example, an alumina porous body having a high-specific surface area and the noble metal is supported on a substrate, for example, a monolithic substrate made of a refractory ceramic or metallic honeycomb structure, or on refractory particles.
On the other hand, the exhaust gas discharged from the diesel engine contains sulfate salts based on sulfur content in a fuel, tar-like particulate matters (referred to as “PM”) derived from incomplete combustion, nitrogen oxide (NOx) or the like.
As an apparatus for removing the PM contained in the exhaust gas discharged from the diesel engine, an exhaust gas purification apparatus, which collects the PM in a diesel particulate filter (referred to as a “DPF”) and burns the collected PM at an appropriate timing to remove, has been well known.
Usually, this DPF is configured such that a porous filter substrate with a honeycomb structure forms a skeleton to collect the PM in a surface of a partition wall of the substrate when the exhaust gas flows inside the partition wall.
In both of a catalyst for purifying the exhaust gas discharged from the gasoline engine and a catalyst for purifying the exhaust gas discharged from the diesel engine, conventionally, expensive noble metals such as platinum (Pt) or rhodium (Rh) have been used as a catalytic activity component in many cases. However, these noble metals are very expensive due to a small amount of reserves and suffer sharp fluctuations in price depending on changes in demand. Therefore, a catalyst containing transition metals, and alumina powder or ceria-zirconia powder has been proposed in the related arts, as a catalyst in which the expensive noble metals are not used or noble metal usage is reduced using other metals different from the noble metals.
For example, Patent Document 1 proposes a catalyst containing a catalyst component consisting of a carrier formed of a ceria-zirconia solid solution and an oxide of at least one metal selected from Cu, Fe, and Mn which is supported on the carrier, as a catalyst capable of improving DPF combustion efficiency.
Patent Document 2 proposes a catalyst in which Cu, Fe, Co, or Ni is supported on a ceria-zirconia solid solution having a pyrochlore structure.
Patent Document 3 proposes a catalyst in which particles consisting of copper or copper oxide are supported on a ceria•zirconia carrier and a ratio between Ce and Zr (Ce/Zr, mass ratio) in a surface of the carrier is in a range of 0.5<Ce/Zr<2.5, as a catalyst capable of improving purification performance of NOx.
Patent Document 4 proposes an exhaust gas purifying catalyst containing a spinel-type composite metal oxide, which is represented by AB2O4, and at least one selected from Al2O3, ZrO2, TiO2, and SiO2.